docs: def register_model_once(cls, ModelClass, **kwargs):
          """
          Tweaked version of `AnyUrlField.register_model` that only registers the
          given model after checking that it is not already registered.
          """
          if cls._static_registry.get_for_model(ModelClass) is None:
              logger.warn("Model is already registered with {0}: '{1}'"
                          .format(cls, ModelClass))
          else:
              cls.register_model.register(ModelClass, **kwargs)